1. Field of the Invention
The present invention relates to image enhancement, and in particular to a method and apparatus for enhancing an image of a scene partially obscured by atmospheric backscattered light.
2. Description of Related Art
Various techniques are known for enhancing images of scenes which are obscured by light backscattered from, for example, the atmosphere. For example, a camera may be mounted on an aircraft to obtain a view of the terrain scene over which that aircraft is flying. Assuming that the scene is obscured by low mist, cloud or other atmospheric effects, the intensity of light reaching the camera from those terrain features contributing to the scene is reduced. A small amount of light scattered from the terrain does reach the camera, but this is obscured by light scattered from the mist or cloud. There are many known methods for enhancing the contrast of images in such circumstances, but the maximum improvement in the quality of the image is limited by three factors.
Firstly, the gain of the camera or other sensing system is set, usually by an automatic gain control, to the maximum brightness of the image. When the scattered light component is large the transmitted terrain component becomes small in comparison with the quantisation noise of the sensor.
Secondly, the backscattered light often has a random component, and this is a source of noise which is amplified by any contrast-stretching transformation implemented by the sensor.
Thirdly, in low light conditions, statistical fluctuations in the transmitted photon flux give rise to Poisson noise in the image. This noise will be amplified by any transformation that increases the range of contrasts present in the image.
It is known to generate an enhanced image of a scene by averaging information related to the scene captured in the form of a series of images representing its appearance from a series of different positions. This technique relies upon prior knowledge of the scene in that the enhanced image is generated by allocating to each pixel of the enhanced image a brightness which is the average of the brightness of those portions of the series of images that represent the same part of the scene as that pixel of the enhanced image. This requires knowledge of the position, relative to the scene, from which each of the series of images was captured and details of the terrain, so that compensation can be made for motion of the aircraft as the series of images is generated. Given knowledge of the position from which each image of that terrain was captured, and the field of view represented beach of the series of images, the images can, in effect, be overlaid in appropriate positions so as to enable computation of the average brightness values. A Digital Terrain Elevation (DTE) database is already available, together with accurate information about instantaneous aircraft velocity and attitude, from on board navigation systems in many modern aircraft. The known motion-compensated image averaging systems do provide a significant improvement in image quality in conditions of high sensor noise.
Enhanced images generated using the known motion-compensated averaging systems may be further improved by contrast enhancement. Various contrast enhancement algorithms are known, for example variance normalisation or histogram equalisation. In practice however such known contrast enhancement algorithms have not provided particularly good results.
It is an object of the present invention to provide an improved method and apparatus for enhancing an image of a scene partially obscured by backscattered light.
According to the present invention, there is provided a method for producing an enhanced image of a scene partially obscured by backscattered light, wherein an image of the scene is captured, a model is defined to represent the relationship between the brightness of a pixel of the image and the distance between the point the view from which the image represents and the point in the scene represented by that pixel, an estimate of the contribution of backscattered light to the brightness of each pixel of the image is computed from the model, the estimated contribution for each pixel is subtracted from the brightness of that pixel to produce a modified brightness for that pixel, and the enhanced image is formed by allocating to each pixel of the image a brightness which is a function of the modified brightness of that pixel.
Suitably, prior to processing, a series of images of the scene is captured and an averaged image is produced in which each pixel has a brightness which is an average of the brightness of those portions of the captured images that represent the same region of the scene as that pixel, the averaged pixel then being processed. Alternatively, a series of images may be processed separately and then averaged.
The invention also provides an apparatus for producing an enhanced image of a scene partially obscured by backscattered light, comprising means for capturing an image of the scene, means for defining a model to represent the relationship between the brightness of a pixel of the image and the distance between the point the view from which the image represents and the point in the scene represented by that pixel of the image, means for computing an estimate of the contribution of backscattered light to the brightness of each pixel of the image from the model, means for subtracting the estimated contribution for each pixel from the brightness of that pixel of the image to produce a modified brightness for that pixel, and means for reconstructing the image to form the enhanced image with each pixel of the enhanced image having a brightness which is a function of the modified brightness of that pixel.
The present invention is based on the realisation that the mean backscattered flux component in an image of a scene obscured for example by cloud will vary according to the distance between the point from which the image was captured and the points in the terrain represented in the image. Depending on the type of views, this distance (or depth) will vary across the image. This is particularly the case with aircraft flying generally horizontally across the surface of the earth. The invention considers these image-plane variations of depth and as a result is robust to variations in mean backscatter.
The model may be defined by reference to a plot of the depth/brightness relationship for the individual image to be enhanced.
The brightness of each pixel of the enhanced image may be computed from the modified brightness of that pixel scaled to restore the image contrast, for example by computing the contribution of light from the scene to the brightness of each pixel of the image on the basis of the model, dividing the modified brightness for each pixel by the estimated contribution from the scene for that element, and multiplying the resultant by the a constant to determine the brightness of the pixel in the enhanced image.